Digital television systems transmit television channels to the viewer in digital, rather than analogue, form. The digital channels are encoded into a digital data stream at the transmitter end, and are decoded at the receiver end using a digital receiver/decoder. To allow interactivity, an uplink may be provided, either via the same medium that delivers the television channels, or else via a different medium such as a telephone link. Further types of data, such as digital audio, software and interactive data can be or are also broadcast. As used herein, the term “digital television system” includes for example any satellite, terrestrial, cable and other system.
The term “receiver/decoder” as used herein may connote a receiver for receiving either encoded or non-encoded signals, for example television and/or radio signals, preferably in MPEG format, which may be broadcast or transmitted by some other means. The term may also connote a decoder for decoding received signals. Embodiments of such receiver/decoders may include a decoder integral with the receiver for decoding the received signals, for example, in a “set-top box”, such as a decoder functioning in combination with a physically separate receiver, or such a decoder including additional functions, such as a web browser, a video recorder, or a television.
The term MPEG refers to the data transmission standards developed by the International Standards Organisation working group “Motion Pictures Expert Group” and in particular but not exclusively the MPEG-2 standard developed for digital television applications and set out in the documents ISO 13818-1, ISO 13818-2, ISO 13818-3 and ISO 13818-4, and the MPEG-4 and other contemplated MPEG standards. In the context of the present patent application, the term includes all variants, modifications or developments of MPEG formats applicable to the field of digital data transmission.
In known receiver/decoders, a releasably connectable smartcard provides secure storage of cryptographic ‘secrets’ (such as the master and other exploitation keys required to decode conditional access data), and also performs critical cryptographic operations in respect of the cryptographic ‘secrets’. Typically, a dedicated smartcard reader is also used, providing a standard (if inflexible) interface to other software and hardware components of the receiver/decoder which require use of the smartcard.
In other developments, systems have been proposed for receiver/decoders to allow the storage (and later playback) of received audiovisual content on mass storage devices attached to or contained within the receiver/decoders. Many such systems store the audiovisual content in scrambled form for security reasons, and—principally for security reasons—use smartcards, such as those described above, to descramble the content during playback (in a similar way to the way in which the smartcards are used during live viewing of content).
The present invention seeks to address problems identified in the above prior art.